1. Field of the Invention
The present invention is broadly concerned with continuous mixing apparatus for the gentle yet thorough mixing of incoming product streams to yield a final product stream of predetermined desired characteristics. More particularly, the invention is concerned with such mixers, rotatable screws used therein, and methods of operation thereof, permitting the mixers to be used in overall systems preferably designed for the mixing of dissimilar characteristic incoming meat streams to produce a final product output stream of substantially constant characteristics. In addition, the mixer may also be used in the processing of substantially homogeneous products, such as in the mixing and stretching of cheese curd and the blending of fruits.
2. Description of the Prior Art
The meat industry is increasingly concerned with xe2x80x9ccase readyxe2x80x9d meats which are prepared and packaged at a central processing facility, ready for display and sale at supermarkets. This marketing approach minimizes costly on-site labor at the supermarkets, leading to lower consumer prices. For example, comminuted meat products (e.g., hamburger) can be produced at a central facility and packaged in convenient sized consumer packages. One difficulty in this approach, however, lies in providing a consistent comminuted product having, e.g., the same fat/lean ratio. This is particularly difficult owing to the fact that starting meat sources may have very different fat/lean ratios, on a day-to-day or even hour-to-hour basis. Hence, a plant may be provided with meats having two widely divergent meat sources in terms of fat/lean ratios or other characteristics, and must be capable of accommodating such staring materials while still producing a comminuted product of substantially constant final properties.
Meat comminuting and mixing devices are in general well known, ranging from simple household sausage grinders to large industrial equipment. However, such prior devices cannot properly handle diverse starting products while still yielding consistent final products. In addition, it is important in the mixing and handling of meat products that the meat not be comminuted and mixed to the point that it exhibits xe2x80x9csmearingxe2x80x9d or the loss of particulate appearance.
There is accordingly a need in the art for improved mixing apparatus and systems which can accept starting products of divergent and changing characteristics while nevertheless producing final products having predetermined, consistent properties; in the context of meat processing, such mixing apparatus must also accomplish these ends without significantly altering the desired meat appearance.
The present invention overcomes the problems outlined above, and provides a continuous mixer for mixing incoming product streams to yield a final product stream of desired characteristics. Broadly speaking, the mixer includes an elongated housing having a plurality of inputs for receiving incoming product streams, as well as an output for the final product. A plurality (usually two) of elongated, side-by-side, axially rotatable mixing screws are located within the housing and extend along the length thereof in order to convey and mix the incoming streams and to move the final product out the housing output. Each of these mixing screws includes a series of outwardly projecting mixing elements each having a base and a plurality of converging sidewall surfaces, the latter cooperatively defining an outer end having a surface area less than the base surface area. These mixing elements are oriented along the length of the mixing screws, preferably in a helical pattern.
In preferred forms, the mixing screws are in intermeshing relationship and are designed to co-rotate, i.e., to rotate in the same direction; however, the screws can also be counter-rotating if desired. The screws preferably include input sections adjacent the housing product stream inputs and present helical flighting along the lengths thereof; the screws also have output sections extending from the ends of the input sections toward the housing output, with the outwardly projecting mixing elements being located on the output sections. The individual mixing elements are generally pyramidal in shape, presenting a base of generally diamond-shaped plan configuration with four outwardly extending, arcuate converging wall surfaces terminating in an apex-like outer end.
The housing is equipped with a plurality of injection ports spaced along the length thereof to permit injection of materials such as CO2 into the housing during operation. In addition, the housing also a series of sensor ports along the length thereof to permit installation of temperature, pressure or other parameter sensors. In order to provide better temperature control, the housing has an outer shell and inner screw-receiving walls to define therebetween a passageway; cold water or other cooling media may be circulated through the passageway during operation of the mixer.
A particular (although not exclusive) utility of the mixer of the invention is for producing a comminuted meat product having a predetermined and substantially constant fat/lean ratio, using input meat streams of different fat/lean ratios respectively. To this end, the mixer is preferably used in an overall mixing system including a plurality of incoming product lines operably coupled with the mixer, where each of the product lines has a product source, a product pump and a product analyzer. In such a system, the pumps are operated to generate the incoming product streams, which are analyzed to determine a desired input characteristic thereof (such as fat/lean ratio). The operational speed of the individual product line pumps is then adjusted in response to analysis of the incoming product streams, thereby generating product streams having a desired input characteristic at a substantially constant magnitude for each incoming product stream. Once such constant characteristic streams are achieved the incoming product streams are directed to the mixer which is operated to create the final product stream. Preferably, this final product stream was again analyzed to determine a desired output characteristic thereof, followed by altering the operational speed of one or more of the product line pumps as necessary to maintain the desired output characteristic in the final product stream.
In another embodiment, each of the incoming product lines includes a combined preblender and pumping device in lieu of separate preblenders and pumps. Such a combined device preferably includes an upper hopper equipped with a pair of rotatable shafts having outwardly extending paddle elements. The hopper presents a bottom wall having front and rear portions, opposed front and rear wall sections and opposed sidewall sections coupled to the bottom wall. There is an opening through the bottom wall adjacent the front portion thereof. The hopper defines a mixing zone along the length of the bottom wall rear portion which is separate from the pumping device housing for initial mixing of an incoming product stream in the mixing zone with subsequent delivery of the mixed incoming product stream to the housing through the bottom wall opening and housing input. Also the device includes lower pumping section made up of side-by-side, fully intermeshed, rotatable auger screws which create the necessary pumping force to convey product from the combined device for downstream processing.
The preferred mixer is designed so as to mix incoming product streams and create a homogeneous output of substantially constant characteristics, without undue meat comminution or smear. In practice, the mixers of the invention are operated so as to limit meat temperature to no greater than about 50xc2x0 F., more preferably from about 20-40xc2x0 F. Residence time in the mixers of the invention should range up to about 3 minutes, more preferably from about 1-2 minutes; pressure conditions within the mixer are essentially atmospheric, but the mixer may be operated at a slight positive pressure if desired.
While the system and continuous mixer of the invention are especially adapted for use in the meat industry, a number of variations are possible. For example, spices or liquid smoke may be injected into the continuous mixer to produce sausage-like products. Alternately, textured vegetable protein may be added to one or more of the meat streams, or the system can be used to mix a meat stream and a TVP stream, respectively. Finally, the mixer of the invention, owing to its unique screw configuration, may be used for the processing of non-meat products such as cheeses, fruits and vegetables.